For the most part, scientists have come to terms with the existence of an unknown antigravity force permeating the cosmos. This "dark energy" — a conveniently ambiguous term for something no one understands — sticks its nose into cosmology on a regular basis and, increasingly, won't be denied.
While we're nowhere near cracking dark energy's secrets, a team of astronomers from the University of Hawaii's Institute for Astronomy has confirmed its effects on the microwave background radiation we see from the early universe. The team's data also confirm theories that large-scale cosmic structures — shaped in part by dark energy — should give rise to anomalies in this radiation.
The astronomers, led by István Szapudi, looked for what's called the late-time integrated Sachs-Wolfe (ISW) effect. It's a lot of words to describe something relatively straightforward:
Imagine a rubber sheet stretched taut. If you take, say, five dinner plates and set them close to each other on the sheet, they create a deep valley. If instead you spread the plates farther out on the sheet, they'll make a shallower valley.
Now add the astronomy: the plates are the galaxies of a gigantic supercluster 500 million light-years across. The sheet is space-time, and the galaxies in it move apart from each other because space-time is expanding like stretched rubber. (That's what astronomers mean by "expansion of the universe.") Dark energy speeds up the rate of this expansion.
A photon from the far background travels toward you though space-time like a marble rolling on the sheet. It falls down one side of the supercluster's valley, thereby gaining a little energy. In a non-expanding universe, the photon would use up that same amount of energy when it climbed the opposite side, with no net effect.
But in an expanding universe, space-time stretches and the supercluster's valley flattens out during the photon's 500-million-year journey across the valley. When the photo arrives at the other side, the hill it climbs up is shorter than the hill it first went down. So the photon keeps some of the energy that it gained when falling in. This difference appears as a temperature increase — in this case, a change of ninety millionths of one kelvin (i.e. really really small).
On the other hand, if the photon first climbed up a hill — a region with a below-average number of galaxies such as a supervoid — that hill would be lower by the time the photon came back down. The photon would never regain all the energy it lost by climbing. In this case, the photon would be slightly colder.
That's the late-time integrated Sachs-Wolfe effect.
The Hawaii team studied this effect on microwaves that passed through 50 superclusters and 50 supervoids mapped at various places on the sky by the Sloan Digital Sky Survey. The microwaves come from the cosmic microwave background (CMB) radiation — the blotched-looking image at right that is our earliest picture of the universe, originating when matter and light separated a mere 380,000 years after the Big Bang.
Because temperature fluctuations existed in the CMB even before the radiation passed through later superstructures, the astronomers had to find a way to reveal the ISW effect hiding in this "noise." They did so by stacking CMB images of the sky that correspond to superstructures' locations.
"Each time you add another image to the stack, the CMB fluctuations average out, thus get smaller, and our desired ISW signal gets stronger," explains Szapudi. Summing up the stacks, the scientists found that slightly warm and cool spots on the microwave background indeed line up with superclusters and supervoids, respectively. The spots' sizes and strengths across cosmic ages match what accelerating expansion predicts.
Scientists have studied the ISW effect before, and the Hawaii group's results bring us no closer to understanding dark energy's nature, says Mario Livio (Space Telescope Science Institute). Still, the study supports other teams' work, particularly theories that the prominent "Cold Spot" — a (you guessed it) very cold region on the CMB discovered in 2004 — results from a supervoid (still unconfirmed, but more likely now). And the further evidence for dark energy's existence may be a solid step toward constraining current cosmological models, notes Sean Carroll (Caltech).
The paper, lead-authored by Benjamin Granett in collaboration with Szapudi and Mark Neyrinck, will appear in a future issue of the Astrophysical Journal Letters.